Frequency-based wheel speed sensors such as tachometers are well known. However, at low speeds, the signal output by these sensors becomes unusable. These speed sensors, when used on aircraft, for example, often do not produce a usable signal at speeds below 5 to 15 knots.
The outputs from these sensors may be used to control deceleration or differential wheel deceleration at high speeds or to control anti-skid brakes, for example. At low speeds, when deceleration is nearly complete and when skidding is unlikely to occur, it is generally not critical to obtain accurate speed information. However, in certain applications, those involving unmanned vehicles, for example, a wheel speed sensor that produces a usable signal down to and including zero speed would be highly desirable or necessary. The output of such a sensor would enable a controller to determine whether the vehicle is at rest or slowly rolling off a runway, for example, when there is no pilot in the vehicle to make such a determination.
Resolvers are generally used to measure the position of a rotating member, such as a motor shaft attached to a rotor, with respect to a fixed element such as a stator. A resolver is essentially a rotary transformer designed so that the coefficient of coupling between the rotor and stator varies with the relative positions of the rotor and stator. Usually resolvers have two windings on a rotor and two windings on a stator positioned at right angles to each other. When a rotor winding is excited with an AC reference signal, the stator windings produce AC voltage outputs that vary in amplitude according to the sine and cosine of rotor position. The stator signals from the resolver are input to a specialized A/D converter known as a resolver-to-digital (R/D) converter. The R/D converter multiples input sine and cosine values by cosine and sine terms, forms the difference and drives its output angle until a null value is reached. The output of the R/D converter is a digital signal that indicates the position of the rotor with respect to the stator and may vary, for example, from 0 to 1 over the course of a single rotation.
Some R/D converters, including models AD2S80A and AD2S83A made by Analog Devices of Norwood, Mass., can also produce an output voltage proportional to velocity. A resolver that outputs an analog position signal is discussed in U.S. Pat. No. 5,260,650, the entire contents of which are hereby incorporated by reference. However, these analog signals do not provide velocity information down to zero speed and are limited in their precision.
It would therefore be useful to provide a vehicle wheel sensor that is usable over a wide range of vehicle speeds including zero speed and that does not suffer from the foregoing problems.